Score a number (6)
I believe the answer is:
twenty
This clue is a double definition.
'score' is the first definition.
(synonyms)
'a number' is the second definition.
(I know that twenty is a type of number)
(Other definitions for twenty that I've seen before include "previous clue?" , "Cardinal" , "One score" , "made by Q on a light blue square" , "Number" .)